1. Field of the Invention
The present invention relates to a mirror driving apparatus, and in particular, to a structure of a micro mirror device suitable for an optical deflector used in light scanning, a method of driving such a micro mirror device, and technology for manufacturing the same.
2. Description of the Related Art
Characteristic features of a micro scanner which is manufactured using silicon (Si) fine processing technology (hereinafter, called a “MEMS (Micro Electro Mechanical System) scanner”) are its compact size and reduced power consumption compared to a polygon mirror which is a conventional light scanning module. Therefore, a MEMS scanner is expected to have a broad range of application from laser projectors to optical diagnostic scanners such as an optical coherence tomograph (OCT), or the like.
There are various methods of driving a MEMS scanner, but of these, a piezoelectric drive method using deformation of a piezoelectric body is promising as a method for obtaining a large scanning angle with a compact system, since this method has large torque but involves a simple device structure and a simple drive circuit. The structure of a general piezoelectric micro mirror (piezoelectric MEMS scanner) is described below.
<Structure of Torsion Bar>
A general piezoelectric MEMS scanner often employs a structure in which a torsion bar aligned with the rotational axis is connected to a mirror, and the torsion bar is twisted by a plurality of piezoelectric cantilevers (see Japanese Patent Application Publication Nos. 2008-257226, 2009-169089 and 2005-177876). FIG. 14 shows the structure depicted in “FIG. 7” of Japanese Patent Application Publication No. 2008-257226. Reference numeral 01 in FIG. 14 is a mirror section, 02a and 02b are torsion bars, and 03a to 03d are piezoelectric cantilevers.
FIG. 15 shows a schematic cross-sectional diagram of a torsion bar in FIG. 14 viewed in the axial direction. Here, the torsion bar labeled with reference numeral 02b is shown, but the same applies to the other torsion bar 02a. As shown in FIG. 15, in this structure, two piezoelectric cantilevers 03b, 03d disposed on either side of the torsion bar 02b are driven so as to be displaced respectively in opposite up/down directions and to twist the torsion bar 02b. 
A torsion bar driving method has a problem in that not all of the force produced by the piezoelectric cantilevers is used to rotate the mirror and hence the use efficiency of the force is poor. In other words, the pure torque for twisting the torsion bar which twists about a certain axis of rotation is produced by a couple about a twisting axis (a vector component in the A direction). However, in the twisting movement produced by the piezoelectric cantilevers 03b, 03d such as those shown in FIG. 15, as the amount of displacement of the to cantilevers increases, so the direction of action of the force moves away from the couple about the twisting axis.
The couple component required in order to twist the torsion bar 02b is the vector direction indicated by A in FIG. 15. This couple component in direction A is obtained as the A-direction component of the cantilever force (the vector indicated by C in FIG. 15). The cantilever force (C-direction vector) is the direction in which the front end portion of the lever bends, in other words, a direction perpendicular to the surface of the front end portion, and therefore the greater the displacement of the cantilever, the smaller the couple component (A-direction component) which contributes to twisting of the torsion bar, and the greater the component of the surplus force which is unrelated to twisting (the component in the pulling direction indicated by B in FIG. 15). Hence, there is a drawback in that even if a large bending displacement is obtained in the piezoelectric cantilevers, this displacement cannot be reflected efficiently in the amount of twisting of the torsion bar, in other words, the amount of tilt of the mirror section.
As stated previously, in a torsion bar structure, when the torsion bar is twisted by piezoelectric cantilevers, a force pulling in a direction perpendicular to the axis of rotation (a B-direction vector component) is generated, and this force produces a dynamic energy loss and leads to decline in the angle of rotation. Since this energy loss becomes greater, the higher the angle of rotation, then it is extremely difficult to achieve a large angle of rotation with a structure based on a torsion bar.
<Bending Hinge Structure>
A MEMS scanner device 410 having a structure such as that shown in FIG. 16 is considered to be a solution for the aforementioned problems of a torsion bar structure. This structure is one in which piezoelectric cantilevers 414 are connected to end portions 412A of a rectangular mirror section 412 via thin plate coupling sections 416. Base end portions 414B of the piezoelectric cantilevers 414 are fixed to a fixing section 430 which forms a fixing and supporting member. By causing the end portions 412A of the mirror section 412 to oscillate up and down by upward and downward driving of the piezoelectric cantilevers 414, a tilting motion is induced in the mirror section 412 due to an inertial force. A structure of this kind is called a “bending hinge structure”.
The bending hinge structure induces resonance in the direction of tilt (rotation) by oscillation of the end portions 412A of the mirror section 412, and tilts the mirror by this resonating oscillation. By providing spiral-shaped coupling sections (hinges which are bent in a meandering shape) 416 between the mirror sections 412 and the piezoelectric cantilevers 414, the displacement of the tilt angle of the mirror sections 412 is further increased. However, the spiral-shaped (meandering) coupling sections 416 are not an indispensable element and it is also possible to adopt a composition in which the piezoelectric cantilevers are connected directly to the end portions of the mirror section 412.
FIG. 17 shows a schematic drawing of movement based on a bending hinge structure. In FIG. 17, in order to simplify the description, a composition is shown in which piezoelectric cantilevers 414 are connected directly to the end portions of the mirror section 412. In FIG. 17, the direction of acceleration of the piezoelectric cantilevers 414 (the direction in which force is applied) is the downward direction indicated by arrow D, and the direction in which the inertial force applied to the mirror section 412 acts is the opposite direction (the direction of arrow E). In other words, this inertial force coincides completely with the direction in which the mirror section 412 is tilted, and no surplus force component is generated. Therefore, the force generated by the piezoelectric cantilevers 414 is used efficiently as a force for tilting the mirror section 412.
In this hinge bending structure, when the piezoelectric cantilevers 414 are driven at the resonance frequency of the rotational motion of the mirror, as shown in FIG. 18, the hinges (coupling sections 416) bend with the displacement of the cantilevers, and a rotational motion resonance of the mirror section 412 is induced due to the generation of an inertial torque in the mirror section 412.
As illustrated in FIG. 17, the direction of displacement of the piezoelectric cantilevers 414 substantially coincides with the direction of rotation of the mirror section 412 at all times, and all of the force is used to rotate the mirror. Therefore, the use efficiency of this force is clearly better than a torsion bar structure, and even if the angle of rotation is large, a large displacement is obtained since there is little energy loss. Furthermore, if meandering coupling sections 416 such as those shown in FIG. 16 are employed in the coupling sections between the mirror section 412 and the piezoelectric cantilevers 414, then the plurality of bending hinges in the meandering shape bend little by little, thereby accumulating displacement, and hence a merit is obtained in that little stress is applied to each individual hinge and the coupling sections are not liable to break, even with a high angle of rotation.
<Meandering Piezoelectric Cantilever Structure>
The structure proposed by M. Tani, M. Akamatsu, Y. Yasuda, H. Toshiyoshi, in ‘A two-axis piezoelectric tilting micro mirror with a newly developed PZT-meandering actuator’ in ‘MicroElectro Mechanical Systems, 2007. MEMS. IEEE 20th International Conference (2007), pp. 699-702, is a structure which is close to the bending hinge structure described above. However, this structure does not employ resonance driving, but rather the cantilevers themselves are formed in a meandering shape folded in a plurality of layers and are disposed on either side of a mirror. In this method, the displacement is increased by adopting a folding structure for the piezoelectric cantilevers and driving the cantilevers so as to induce alternating bending in opposite directions, whereby the mirror can be tilted by a large amount even without using resonance.